I believe the only countries with cyan 920s are US and Australia.
The US model is the RM-820 (different to uk one)
However the Aus model is the RM-821 (same as uk one)

Unfortunately the cyan 920 is exclusive to Telstra (a carrier).
Fortunately all Telstra 920s come unlocked straight out of the box and can be bought outright without a contract............EXCEPT FOR THE CYAN ONE!!!

Got myself a white 920 and went to a Nokia care centre, was told that a case change could be done. Did not ask price but was assured that this will not affect warranty of the device.

^Yep, they do this. In Australia the exact price is $118 (including shipping to and from the centre, so if you can actually walk to one, itll probs be cheaper). Also i'm sure the pricing varies by country.

Buy the casing and change it out yourself. Someone installed a yellow case on their originally black 920. Search for it.
It's pretty easy to swap out the case, the only somewhat tricky part is the inductive charging coil.
Now how difficult it is to find cyan housing I do not know.

I actually managed to bag myself a genuine matt cyan housing cover on eBay last week. It arrived on Monday. Once I received it, I went to the local nokia care shop who agreed to fit it for £30. The engineer checked over it and compared it to one of their cover's and he confirmed it was genuine and was impressed I got hold of it. I am now one of the lucky few to have a cyan lumia 920 in the UK :)